Piston with a reinforcing insert

ABSTRACT

Piston with a reinforcing insert, presenting a top face (11) with a recessed combustion chamber (12) provided with a lateral peripheral wall, defined by the annular insert (20), which presents an internal end face (24) with a radial extension (d 1 ) that is substantially smaller than the radial extension (d 2 ) of the opposite external end face (22) of the annular insert (20). The radial thickness of the insert (20) varies along the profile of its external peripheral face (23), in order to maintain a value necessary to meet the requirements of structural reinforcement of the piston (10), at the region where the insert (20) is applied.

FIELD OF THE INVENTION

The present invention refers, in general, to a new construction forpistons used in internal combustion engines and, more particularly, to anew construction for that type of pistons incorporating a reinforcinginsert, usually called composite, in order to increase the strength ofthe piston at those portions that are submitted to higher stresses.

BACKGROUND OF THE INVENTION

The high thermo-mechanical stress to which the piston of more advancedengines are submitted led to the need of incorporating localreinforcements, in order to maintain the structural integrity of thealuminum pistons.

Particularly at the region defined by the combustion chamber edge, wherethe highest temperatures and thermo-mechanical stresses take place, inorder to increase the local strength of the aluminum alloy of saidpistons, it is common to incorporate porous ceramic inserts that areinfiltrated by the aluminum base alloy. As a result from themanufacturing process used for obtaining said insert, usually thesqueeze casting process for aluminum alloys, a composite of highstructural strength is produced.

Due to the characteristics of the reinforcing material, the thermalexpansion of the composite material is inferior to that of the basematerial of the piston (aluminum alloy).

In case the insert presents the usual annular shape defining theperipheral wall of the piston combustion chamber, a serious problemoccurs at the interface between the low expansion material of the insertand the piston remaining material made of aluminum alloy of higherexpansion. Due to the difference between the thermal expansioncoefficients of both materials, there occur stresses, resulting from thedifference between the expansion of both materials under the sametemperature.

Since the insert is in the form of a ring, which presents a generallyrectangular cross section, which is disposed around the pistoncombustion chamber and which is projected practically throughout thewhole height of the latter, from the upper end face of said piston, theinterface defined between the lower end face of the insert and theadjacent radial face of the piston material made of aluminum alloy isdisposed at a region submitted to high temperatures, as said region isclose to the bottom of the piston combustion chamber.

The high temperatures at the piston region defined along the radialextension of said lower interface of the insert tend to force the pistonaluminum alloy to expand radially, at values different from those of thecorresponding radial thermal expansion of the insert material, thusdemostrating the occurrance of the stresses mentioned above, which reachvalues sufficient to cause cracks and even ruptures on the pistonaluminum alloy.

DISCLOSURE OF THE INVENTION

The present invention has the objective of providing a new constructionfor a piston of this type, which avoids or at least minimizes theoccurrance of the above cited stresses, without affecting the structuralintegrity of the piston.

The piston object of the present invention is generally cast in aluminumalloy, presenting a top face provided with a recessed combustionchamber, which has a peripheral lateral wall defined by the annularinsert. According to the invention, the annular insert presents aninternal end face with a radial extension that is substantially smallerthan the radial extension of an opposite external end face of theannular insert, the radial thickness of the annular insert varying alongthe profile of the external peripheral face, in order to maintain avalue required for the structural reinforcement of the piston at theregion where the insert is applied.

With the constructive arrangement cited above, the insert may present aportion of its extension, adjacent the external end face, with a largerthickness, which is necessary for achieving the desired structuralreinforcement of the piston at this end face region, which surrounds thecombustion chamber and which is subjected to high thermal and mechanicalefforts, and also to high temperatures. As there is not a radialinterface between the insert and the remaining portion of the piston atsaid external end portion of the insert, the value of the thickness ofthe latter will not cause relevant stresses to the piston, resultingfrom the difference of thermal expansion of both materials at the sametemperature.

It should also be observed that the axial interface region, disposedbetween the insert and the piston aluminum alloy and close to the topface of the latter, is subjected to much lower temperatures than thoseof the radial interface region adjacent the bottom of the combustionchamber. Nevertheless, at the radial interface region, the piston issubmitted to mechanical efforts, which are not as high as those existingat the external end portion of the insert, thereby not requiring topresent the same thickness as the external end portion.

The reduction of the radial thickness of the insert, according to therequirements of the structural reinforcement of the piston, allowsachieving a radial interface of reduced length, thereby practicallyavoiding the occurrance of thermal stresses originated from thedifference between the thermal expansion coefficients of the compositeand the remaining material of the piston, which are relevant andprejudicial to the structure of the piston at that region.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, with reference to the attacheddrawings, in which:

FIG. 1 is an axial sectional view of the upper portion of a piston,including a reinforcing insert according to the present invention; and

FIG. 2 shows an enlarged detail of FIG. 1, for a better visualization ofthe new construction.

BEST MODE OF CARRYING OUT THE INVENTION

According to the figures cited above, the piston 10, which is usuallymade of aluminum base alloy, presents its top face 11 provided with arecessed combustion chamber 12, and its peripheral face provided withgrooves 13 for piston rings.

The lateral peripheral wall of the combustion chamber 12 is defined byan insert 20, which presents a height or axial extension "H", made ofalumina fibers, for example, presenting the form of a ring, with theinternal peripheral face 21 coinciding with and defining the lateralperipheral wall of the combustion chamber 12, whereas its external endface 22 of radial extension d₂ coincides with and defines the piston topface portion 11, which surrounds the edge of the combustion chamber 12.

The external peripheral face 23 of the insert 20 is defined by acylindrical portion 23a, with the height "h" and adjacent the externalend face 22, and by an inverted frusto-conical portion 23b, whose largerand smaller faces match, respectively, through curvature radii, with thecylindrical portion 23a and an internal end face 24 of radial extensiond₁, facing the bottom of the combustion chamber 12.

As observed in the figures, the radial extension of the internal endwall 24 of the insert 20 is substantially smaller than the radialextension of the external end wall 22. This arrangement of reducing theradial thickness of the insert 20 at its internal end portion allowseliminating the problem of undesirable stresses occurring at that regionof the piston, as a function of the difference between the coefficientsof thermal expansion of the aluminum alloy and the insert.

It should be observed that, with said constructive arrangement, theinternal end face 24 of the insert 20 defines an extension of radialinterface with the base alloy of the piston, said extension being muchreduced at a region submitted to the high temperatures of the combustionchamber and to the mechanical efforts, which are substantially lowerthan those to which is submitted the piston region adjacent the top face11. The reduction of the radial thickness of the insert 20 may be madewith different profiles for its external peripheral face 23, theillustrated face being only a preferred form, in which the followingapproximate dimensional relations are obeyed:

H=(0.09-0.150)×piston diameter

h=(0.03-0.05)×piston diameter

d₁ =d₂ ×(0.30-0.50)

d₂ =(0.04-0.12)×piston diameter

The incorporation of the insert 20 may be obtained through any process,such as squeeze casting the aluminum alloy onto the insert.

After the incorporation of the insert to the base alloy, the compositematerial is machined to the piston final form, as illustrated in thefigures.

We claim:
 1. Piston with a reinforcing insert, said piston presenting atop face (11) with a recessed combustion chamber (12), whose lateralperipheral wall is defined by an annular insert (20), which has aninternal peripheral face (21), an external peripheral face (23), and aninternal end face (24) with a radial extension (d₁) substantiallysmaller than the radial extension (d₂) of an opposite external end face(22), adjacent to and defining part of the piston top face (11),characterized in that the external peripheral face (23) of the insert(20) is defined by a first portion (23a) adjacent to the external endface (22) and mantaining the radial thickness of the insert at a value,necessary to meet the requirements of the structural reinforcement ofthe piston (10) throughout the axial extension of its region where saidfirst portion (23a) is positioned, and by a second invertedfrusto-conical portion (23b), whose smaller base defines the internalend face (24) of the annular insert (20).
 2. Piston, according to claim1, characterized in that the first portion (23a) of the externalperipheral face (23) of the annular insert (20) is cylindrical. 3.Piston, according to claim 1, characterized in that the axial extension(h) of the first portion (23a) of the external peripheral face (23) ofthe insert (20) is from 3 to 5% the diameter of the piston (10).
 4. Apiston according to claim 1, wherein the radial extension of theinternal end face of the insert presents a value of from 30 to 50% ofthe value of the radial extension of its external end face.
 5. Piston,according to claim 4, characterized in that the external end face (22)of the insert (20) presents a value from 4 to 12% the diameter of thepiston (10).
 6. A piston according to claim 1, wherein the axialextension of the insert corresponds to from 9 to 15% of the diameter ofthe piston.